Others hypothesized diet beverages could boost the drinker's preference for sweet tastes, translating to more munching on high-calorie desserts.

"Artificial sweeteners are a lot sweeter than regular sugar, on the order of 250 times sweeter, so that's where the concerns came from," said Vasanti Malik, a nutrition researcher from the Harvard School of Public Health in Boston.

The new findings may ease some of those worries, according to researchers led by Carmen Piernas, from the University of North Carolina at Chapel Hill Gillings School of Global Public Health. But they don't prove pounding back diet drinks is harmless.

The study included 318 overweight or obese adults in North Carolina, all of whom said they consumed at least 280 calories' worth of drinks each day.

Piernas and her colleagues advised one-third of the participants to substitute at least two daily servings of sugary beverages with water. Another one-third was instructed to substitute them with diet drinks, including Diet Coke, Diet Mountain Dew and Diet Lipton Tea.

After three and six months, people reported their food and beverage intake on two different days in detail. A previous publication showed participants in both groups lost weight.

According to the new report, water and diet beverage drinkers reduced their average daily calories relative to the start of the study, from between 2,000 and 2,300 calories to 1,500 to 1,800 calories. At both time points, people in the two groups were eating a similar amount of total calories, carbohydrates, fat and sugar, the research team reported in the American Journal of Clinical Nutrition.

Six months in, the only differences were that members of the water group ate more fruit and vegetables and people randomized to diet beverages ate fewer desserts, compared to their diet habits at the study's onset.

"That's sort of the opposite of what you would expect if consumption of diet soda increased the preference for sweets," Malik, who wasn't involved in the new study, told Reuters Health.

The research was partially funded by Nestle Waters USA, which provided the water used in the study.

Despite the results, diet beverage drinkers may not be totally in the clear.

Another new study, also published in the American Journal of Clinical Nutrition, found Frenchwomen who drank beverages sweetened with either real or fake sugar were more likely to be diagnosed with diabetes over 14 years than those who stuck with water.

'NOT THE FINAL ANSWER'

Piernas cautioned that everyone in her study was heavy and trying to lose weight - so the findings may not apply to normal-weight people who drink a lot of diet beverages.

"This is not the final answer," she told Reuters Health.

Some studies have suggested an increased risk of cancer tied to certain artificial sweeteners, but convincing evidence is lacking, according to Malik.

"We're trying to reduce sugar-sweetened beverage intake in the population for obesity, so the next logical question is: what substitutes can be used?" she said.

"I think (diet drinks) can be consumed in moderation, along with other beverages - water, coconut water, sparkling water, that type of thing," Malik added.